1. Field of the Invention
The present invention relates to an organic light emitting diode (referred to hereinafter as an OLED) display device, and more particularly, to a pixel circuit of an OLED display device which is capable of compensating for a characteristic deviation of a driving thin film transistor and reducing the influence of a previous frame.
2. Discussion of the Related Art
An OLED display device is a self-emissive device in which an organic light emitting layer emits light by recombination of electrons and holes, and is expected to be a next-generation display device in that it is high in luminance, low in driving voltage and ultra-thin in thickness.
Such an OLED display device includes a plurality of pixels, each of which includes a light emitting element having an organic light emitting layer disposed between an anode and a cathode, and a pixel circuit for independently driving the light emitting element. Pixel circuits may be classified into a voltage type and a current type. A voltage-type pixel circuit employs an external driving circuit simpler in configuration than a current-type pixel circuit and is suited to a high-speed operation, so that it may be highly applicable as a pixel circuit for an OLED television (TV).
The voltage-type pixel circuit mainly includes a switching thin film transistor (referred to hereinafter as a TFT), a capacitor, and a driving TFT. The switching TFT charges a voltage corresponding to a data signal in the capacitor in response to a scan pulse, and the driving TFT controls the amount of current to be supplied to an OLED based on the voltage charged in the capacitor to adjust the amount of light to be emitted from the OLED.
However, a conventional pixel circuit has a disadvantage in that the threshold voltage Vth of a driving TFT is non-uniform by positions by reason of a process deviation, etc., resulting in unevenness in luminance, or the threshold voltage varies with time by the same reason, resulting in a reduction in luminance, thereby reducing lifespan. To solve this, the voltage-type pixel circuit has adopted a method of detecting and compensating for the threshold voltage of the driving TFT.
For example, in a conventional pixel circuit disclosed in Korean Patent Laid-open Publication No. 2008-0001482, the threshold voltage of a driving TFT is detected by connecting the gate and drain of the driving TFT with each other through a separate switching TFT, and a data voltage is compensated for by an amount corresponding to the detected threshold voltage. Also, in the conventional pixel circuit, a light-emission switching TFT, connected in series between the driving TFT and an OLED, is used to turn off light emission of the OLED when the threshold voltage is detected.
However, in the conventional pixel circuit, the threshold voltage of the driving TFT can be compensated, but the threshold voltage of the light-emission switching TFT connected in series between the driving TFT and the OLED cannot be compensated, resulting in unevenness in luminance due to a difference in the threshold voltage of the light-emission switching TFT. On the other hand, in order to solve the problem with the light-emission switching TFT, the light-emission switching TFT may be omitted. In this case, the OLED may emit light even in a period other than a light emission period, thereby increasing a black luminance, which may lead to a reduction in contrast.
Also, in the conventional pixel circuit, the gate and source of the driving TFT may be influenced by data of a previous frame, thereby making accurate data input impossible.